1. Field of the Invention
The invention relates to a heat-shielding film formed on a wall surface of a metal base material and a method of forming the same, and relates to, for example, a heat-shielding film formed on a portion or the entirety of a wall surface facing the combustion chamber of an internal combustion engine and a method of forming the same.
2. Description of Related Art
Internal combustion engines such as gasoline engines and diesel engines are constituted mainly by an engine block and a cylinder head, the combustion chamber thereof being defined by the bore surface of a cylinder block, a piston top surface inserted into the bore, the bottom surface of the cylinder head, and the top surfaces of intake and exhaust valves disposed inside the cylinder head. Along with the higher power required of present-day internal combustion engines, reducing the cooling loss thereof becomes important, and one of the measures to reduce this cooling loss is the method of forming a heat-shielding film made of ceramics on the internal wall of the combustion chamber.
However, since the ceramics in general have a low thermal conductivity and a high heat capacity, a drop in the intake efficiency or knocking (abnormal combustion caused by stagnant heat inside the combustion chamber) occur, due to a constant rise in surface temperature. Therefore, they are currently not prevalent as a coating material for the internal wall of a combustion chamber.
Thus, it is desirable that a heat-shielding film on the wall surface of a combustion chamber should be formed of a material that is heat-resistant and heat-insulating while also having low thermal conductivity and low heat capacity. In addition, along with having this low thermal conductivity and low heat capacity, the heat-shielding film is preferably a coat that has deforming capability allowing the film to deform in accordance with the combustion pressure at combustion time, the injection pressure, and the repetitions of thermal expansion and thermal contraction inside the combustion chamber, and is preferably a coat in which peeling is unlikely to occur due to the amount of thermal deformation at the interface with the base material of the cylinder block or the like.
Here, each of Japanese Patent Application Publication No. 2009-243352 (JP-A-2009-243352) and Japanese Patent Application Publication No. 2009-243355 (JP-A-2009-243355) discloses an internal combustion engine having a thin film for heat-insulation in which air cavities are formed inside a material having a lower thermal conductivity than the base material forming the combustion chamber of the internal combustion engine and having a heat capacity that is equal to that of the base material or lower than that of the base material.
Each of the JP-A-2009-243352 and JP-A-2009-243355 above describes a technique for forming a coat with low thermal conductivity and low heat capacity on the internal wall of the combustion chamber of an internal combustion engine, and thus, a heat-insulating film (heat-shielding film) with excellent capability may be formed as described above.
However, since air cavities are formed in the heat-insulating material made of ceramics or the like in the heat-insulating film structure, it is difficult to expect that the heat-insulating film has satisfactory deforming capability. For this reason, in a process in which the heat-insulating film undergoes repeated stresses from thermal expansion and thermal contraction inside the combustion chamber, damages by thermal fatigue may be caused, and furthermore, the thermal deformation difference between the heat-insulating film and the aluminum base material is likely to increase, and peeling is likely to occur at the interface between the heat-insulating film and the base material.